The present invention relates to coating in multiple injection molding part cavities and controlling the flow of an in-mold coating (IMC) composition that is injected into multiple injection molding part cavities, more particularly to a method of (1) injection molding multiple articles in multiple injection molding part cavities and coating the multiple articles with an IMC composition, (2) using only a single injector to coat the multiple molded articles and/or (3) controlling the flow of the injected IMC composition on the multiple molded articles such that the multiple articles are only coated on desired surfaces or portions thereof. The present invention finds particular application as an apparatus and method for injection molding two articles within two-part cavities defined by mold halves of a single injection molding machine, coating the two articles with an IMC composition using a single IMC composition injector and controlling the flow of the injected IMC composition on the two articles.
Molded thermoplastic and thermoset articles, such as those made from polyolefins, polycarbonates, polyesters, polystyrenes and polyurethanes, are utilized in numerous applications including those for automotive, marine, recreation, construction, office products, and outdoor equipment industries. Often, application of a surface coating to a molded thermoplastic or thermoset article is desirable. For example, molded articles may be used as one part in multi-part assemblies; to match the finish of the other parts in such assemblies, the molded articles may require application of a surface coating that has the same finish properties as the other parts. Coatings may also be used to improve surface properties of the molded article such as uniformity of appearance, gloss, scratch resistance, chemical resistance, weatherability, and the like. Also, surface coatings may be used to facilitate adhesion between the molded article and a separate finish coat to be later applied thereto.
Numerous techniques to apply surface coatings to molded plastic articles have been developed. Many of these involve applying a surface coating to plastic articles after they are removed from their molds. These techniques are often multi-step processes involving surface preparation followed by spray-coating the prepared surface with paint or other finishes. In contrast, IMC provides a means of applying a surface coating to a molded article prior to its ejection from the mold.
Molds used with thermoplastics usually are of a “clam shell” design having mated halves that meet at a parting line. One of the mated halves typically remains stationary whereas the other half typically moves between a closed position and an open, retracted position. To form a molded article, the movable half is moved to its closed position and held closed under a clamping force thereby forming a contained molding cavity. Molten material is injected into the molding cavity. The molded article is formed by thoroughly filling the cavity with the molten material and allowing the material to sufficiently cool and solidify. During the entire molding process, the movable mold half is maintained in its closed position. After molding, the mold halves can be opened and a finished, molded article ejected therefrom.
Owing to differences in mold design and molding conditions, processes where the mold is cracked or parted prior to injection of a coating composition generally are not used for the IMC of injection molded thermoplastics. When molding thermoplastics, it is generally necessary to maintain pressure on the movable mold half to keep the cavity closed and prevent material from escaping along the parting line. Further, maintaining pressure on the thermoplastic material during molding, which also requires keeping the cavity closed, often is necessary to assist in providing a more uniform crystalline or molecular structure in the molded article. Without such packing (i.e., pressure maintenance), physical properties of the molded thermoplastic article tend to be impaired.
Because injection molding does not permit the mold to be parted or cracked prior to injection of the IMC composition into the mold cavity, the IMC composition must be injected under sufficient pressure to compress the article in all areas that are to be coated. The compressibility of the molded article dictates how and where the IMC composition covers it. The process of IMC an injection molded article with a liquid IMC composition is described in, for example, U.S. Pat. No. 6,617,033 and U.S. Patent Publication Nos. 2002/0039656 A1 and 2003/0082344 A1.
A method and apparatus used to physically inject liquid IMC composition into the molding cavity of an injection molding machine during the molding process, also referred to herein as a dispense-and-control method and apparatus, is described in commonly owned, copending International Application No. PCT/US03/33186 (WO 2004/041503) the teachings of which relating to that method and apparatus are incorporated herein by reference. The dispense and control apparatus provides a delivery system for injecting an IMC composition into the cavity of a pair of mold halves on an injection molding machine and a means for controlling the delivery system.
As the IMC composition is injected into the mold cavity and onto the molded article, the flow of the IMC composition can be controlled such that only desired surfaces or portions of surfaces of the article are coated and that those surfaces are optimally coated. Further, the flow of the IIMC composition can be controlled so as to limit it from escaping through the parting line or entering the area near the resin injection orifice.
For example, one method for selectively controlling flow of IMC composition flow is described in US 2003/0082344 A expressly incorporated herein by reference, which teaches methods for controlling the flow and thickness of an IMC composition as it is injected into a mold cavity and onto a molded article. Generally, by controlling the thickness or depth of various areas or sections of the molded article, desired areas of the article can be preferentially coated. Specifically, when a molded article is provided with an area of increased relative thickness at or near the location of the IMC composition injection, flow of the IMC composition is promoted. When the molded article is provided with a runner section or preferred flow channel, IMC composition flow over the surface of the molded article is promoted. Additionally, when the molded article is provided with a containment flange, the flange acts as a barrier and prevents the IMC composition from leaking or seeping off a desired surface and/or out of the mold cavity.
Another method for selectively controlling in-mold coating flow is described in US 2003/0077426 A, expressly incorporated herein by reference, which teaches the use of “flow zones” near the IMC composition injection inlet area to promote the flow of IMC composition from the injection inlet area. Still another method for selectively controlling IMC composition flow is described in US 2003/0099809 A, expressly incorporated herein by reference, which discloses a containment flange functioning like the containment flange described in the US 2003/0077426 A publication but with the added feature of being configured to be removable from the coated thermoplastic article. The removable flange is able to be easily removed. Still yet another method for selectively controlling in-mold coating flow is described in US 2003/0077425 A publication, expressly incorporated herein by reference, which discloses the use of a mold structure formed as part of the molded article that provides a barrier preventing IMC composition flow into the resin injector orifice, gate pin assembly, or the like.
In some injection molding applications, multiple articles may be simultaneously injection molded on a single injection molding machine. More particularly, a single injection molding machine can include a set of mold halves that define more than one part cavity. When the mold halves define multiple part cavities, multiple articles can be molded therein, typically one article per part cavity. In these types of applications, it may be desirable to in-mold coat the multiple molded articles for the reasons discussed above. Accordingly, there is a need for an injection molding and IMC arrangement that allows articles molded in multiple part cavities of a set of mold halves to be in-mold coated. It may be further desirable to coat the multiple molded articles using a single IMC composition injector. Whether coating with a single IMC composition injector or with multiple injectors, there is also a need for controlling the flow of the injected IMC composition on the multiple injection molded articles such that they are coated only on desired surfaces or portions of surfaces.